The use of interferometry to measure changes in lengths, distances, and optical paths is well known in industry. Collectively such practice can be termed interferometric displacement measurement. In performing such measurement both homodyne and heterodyne techniques may be used, with the latter having come to be overwhelmingly preferred today. Of present interest are heterodyne techniques using two optical frequencies produced with a single laser device.
In general, in a single mode laser only one frequency of oscillation may be produced. In order to allow more than one frequency to oscillate simultaneously, new boundary conditions have to be introduced into the laser resonator so that more than one gain profile is formed. Each profile then provides gain for a respective polarization of light. The application of a magnetic field to at least part of the laser gain medium is one well known way to accomplish this, and inserting a birefringent material into a laser resonator to create a photoelastic effect producing different optical path lengths for different polarizations of light is another.
When a magnetic field is applied to a single longitudinal mode laser cavity, two oscillation frequencies may be produced which have orthogonal polarizations and are separated in frequency symmetrically with respect to the absolute frequency of the laser (its natural resonant frequency). This is commonly termed the Zeeman effect, and lasers using it are called Zeeman lasers. In a Zeeman laser the magnetic field may be applied along the same direction as the axis of the laser resonator (axially or longitudinally; to avoid confusion with longitudinal mode lasers, "axial" or "axially" are used hereinafter) or perpendicular to the axis of the laser resonator (transversely).
For axial type Zeeman lasers the frequency components produced have opposite circular polarizations and the maximum frequency split is typically a few megahertz, e.g., for He-Ne lasers approximately 4 MHz. [He-Ne lasers are used herein as examples. However it should be appreciated that the Zeeman effect may be obtained in other laser mediums and that the present invention may therefore also use such alternate mediums.] For transverse type Zeeman lasers the frequency components produced have opposite linear polarizations and the maximum frequency split is typically only a few kilohertz, e.g., for He-Ne lasers approximately 300 KHz.
The split dual frequencies obtainable with Zeeman lasers are particularly useful for interferometric displacement measurement using heterodyne techniques. A key benefit is that the Zeeman split is symmetric with respect to the absolute frequency, which can be determined very precisely for the particular laser medium used. It follows that the frequency for each frequency component can also be precisely determined. Zeeman lasers also achieve high signal-to-noise ratios. In interferometric displacement measurement these characteristics permit the interference fringes produced by the motion of a target object to be accurately measured, and the total displacement of the target may then be calculated by integrating the total number of such fringes through time. This method of displacement measuring is accurate and reliable, and has found wide use in industry.
In displacement interferometry the maximum obtainable frequency split imposes a limit on target speed during measurement (velocity=.lambda./2* Doppler frequency). For example, if a measurement target object is moved such that the Doppler effect causes a decrease in the frequency split, the measured frequency can decrease all the way to zero and the interferometer can cease to function. For axial He-Ne Zeeman lasers the maximum target movement rate, commonly called the "slew rate", is approximately 1.2 m/sec. For transverse He-Ne Zeeman lasers the maximum slew rate is considerably less (&lt;0.1 m/sec). Today axial He-Ne Zeeman lasers are widely used in industry, but it is becoming increasingly desirable to perform displacement measurement using still higher slew rates.
One way to increase the frequency split produced by Zeeman type lasers is to apply a stronger magnetic field to the laser resonator. However, there are practical limits to this. As the magnetic field is made increasingly strong, a point is reached at which the gain medium starts to behave in a non-linear fashion, and second order Zeeman effects then cause unwanted modes and frequencies to appear. This confuses the detectors used in interferometer systems. Overly strong magnetic fields also push the gain of the media away from the absolute frequency, dramatically decreasing the laser power produced, until the point at which lasing stops entirely. Thus, there is an upper limit to the frequency split obtainable using the Zeeman effect.
The larger frequency split obtainable with axial type Zeeman lasers is the reason they are primarily used in industry today, despite the disadvantage that they produce circularly polarized light. This is a disadvantage in interferometry because linear polarized light is needed. To convert the output of axial type Zeeman lasers to linear polarizations, quarter-wave plates must be used. However, such conversion is never perfect, producing considerable noise that can severely complicate interferometric measurement. This also results in considerable loss in usable beam power. It should be particularly noted that transverse type Zeeman lasers do not suffer from this inherent disadvantage.
Other techniques than the Zeeman effect can also be used to create multiple frequencies for use in interferometry. One well known example is insertion of a birefringent material into a laser cavity to produce a birefringence. However, such other techniques generally also suffer a common shortcoming: they have a minimum obtainable frequency split of approximately 40 MHz, which is simply not practical for use in most current interferometry applications. Thus, current techniques are not able to produce split dual frequencies for interferometry in a range extending from roughly 4 MHz to 40 MHz.
Since the application of an axial magnetic field imposes a limit in the maximum frequency split obtainable, and the addition of birefringence imposes a limit in the minimum frequency split obtainable, it would seem logical that combining these techniques might produce a frequency split with no range limitations. However, there are physical laws adversely affecting such direct combination. When the Zeeman effect is produced by application of an axial magnetic field, the two frequencies produced are circularly polarized. However, since photoelastic birefringence materials can only produce different optical paths for linearly polarized light, this approach does not work. Alternately, when the Zeeman effect is produced by application of transverse magnetic field, the two frequencies produced are linearly polarized, but the natural anisotropy of the laser system produces a whole new set of problems.
Without a magnetic field, there are also two mutually perpendicular axes from the natural anisotropy of the laser itself, and it is the principal axis of this anisotropy which provides the least loss for an electromagnetic wave. Thus one component occurs in the polarization plane parallel to the principal axis, and saturation of the population drives the other component so that its polarization plane is perpendicular to the principal axis. Because the two components of a single longitudinal mode are strongly coupled with each other, the laser output falls within a locking frequency range, which provides linearly polarized light at a single frequency in some circumstances.
In a transverse Zeeman laser, the applied magnetic field defines the orientations for emitting linearly polarized light, namely a .pi.-component having a polarization plane parallel to the magnetic field and a .sigma.-component having a polarization plane perpendicular to the magnetic field. [The optical convention of referring to horizontal and vertical polarizations as p- and s-components is not used herein. Instead the physics convention of referring to .pi.- and .sigma.-components based upon the direction of the applied field is used, because it avoids confusion with light beam directions.] The polarizations of established light inside a laser are thus not only determined by the applied magnetic field, but also influenced by the natural anisotropy of the laser. The orientation of the linearly polarized laser beam can switch into some directions which are neither the direction of the applied magnetic field, which can easily be controlled, nor the orientations of the anisotropy of the laser, which cannot be controlled. Because it is desirable to use two mutually perpendicular linearly polarized components having a frequency split of several MHz for interferometric measurement, these characteristics of the transverse Zeeman laser prevent it from wide application in present precision measurement applications.
Accordingly, new techniques for achieving split dual frequencies having orthogonal linear polarizations for interferometric measurement are needed, particularly ones which produce frequency splits in the range from 4 MHz to 40 MHz.